Quebec's government has now made another -- and crucial -- move in a strategy directed toward national independence.The challenge to the rest of Canada is explicit. Ever since the separatists came to power in Quebec two years age, they have been committed to a referendum on the negotiation of something that they rather vaguely termed "sovereignty-association." Now they have published their definition of the phrase, and it's vague no longer. It's sovereignity for Quebec, clear and simple. Association means cooperation between two nations, but that's secondary.

The subject isn't likely to be on any agenda when President Carter goes to Canada at the end of this week. There's hardly anything that the United States can say or do that won't make matters worse for its own interest. One sucession might well induce others, and not many Americans are anxious to see the northern half of the continent dissolve into a congeries of small and vulnerable new states. But any forceful expression of American preferences is only going to fuel the regional movements for independence.

Sovereignity, the Quebec provincial government said, means the exclusive power to legislate and to levy taxes. In a referendum next spring, it will ask the voters to authorize it to negotiate that kind of sovereignty with the Canadian federal government in Ottawa. It's a move to which the federal government, under Prime Minister Joe Clark, seems to have no answer so far. Who, after all, will vote against a mandate to negotiate? But it the referendum carries by any significant majority, how can the federal government refuse to parley?

Mr. Clark huffs that the whole idea is "absolutely unacceptable." But he's going to have to do better than that. If he refuses any larger concession to French Canadian concerns, he will only confirm the separatists' charges about English-speaking Canada's obduracy. The issues are the languages in which business is conducted, in which lawsuits are tried and in which children are educated. If the present compromises had seemed satisfactory to most French-speaking Canadians, the separatists would not now be a power.

It would be ironic if Canada broke up just Western Europe, to develop its prosperity and enhance its international power, works toward a federation. The European Common Market -- with nine countries and eight languages -- has now established a directly elected parliament. But examples drawn from other continents aren't likely to carry the Cannadian debate. The separatists have declared their position. The outcome of next spring's referendum will depend on the response by Mr. clark and the federal government.